Measurement systems and methods for determining component particle concentrations in a liquid

ABSTRACT

Methods and apparatus are provided for measuring component concentration in a liquid containing relatively large particles and relatively small particles. The invention may be utilized for measuring fat and casein concentrations in a dairy product, but is not limited to such use. A polarized light beam is directed through a scattering cell having first and second windows and containing a sample of the liquid. A normal to an exterior surface of the second window of the scattering cell is at or near Brewster&#39;s angle with respect to the light beam to reduce or eliminate reflections. A first light detector, positioned at an angle of about 5° to about 45° with respect to the light beam, detects scattered light from a first component of the liquid, such as fat particles. A second light detector, positioned at an angle of about 130° to about 160° with respect to the light beam, detects scattered light from a second component of the liquid, such as casein particles. When the liquid is a dairy product, the detector signals are representative of fat and casein concentrations. Component concentrations of a dairy product may be measured without requiring the use of a chemical diluent.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser.No. 09/507,027 filed Feb. 18, 2000, which claims priority based onprovisional application Serial Nos. 60/120,858 and 60/120,857, bothfiled Feb. 19, 1999.

FIELD OF THE INVENTION

[0002] This invention relates to measurement systems and methods and,more particularly, to measurement systems and methods for determiningcomponent particle concentrations in a liquid containing a mixture ofrelatively large particles and relatively small particles, utilizinglight scattering techniques. The invention is particularly useful fordetermining fat and casein concentration in dairy products, but is notlimited to such use.

BACKGROUND OF THE INVENTION

[0003] It is frequently desirable to determine particle concentration ina liquid. Such measurements may be utilized in process control,research, and the like. In some cases, the liquid may contain particlesof one type having one size distribution. In other cases, the liquid maycontain particles of two or more types, each having a characteristicsize distribution. In the latter case, determining particleconcentrations is difficult because of interaction between measurements.An example of a liquid having two types of particles with different sizedistributions is milk and other dairy products.

[0004] Raw milk prices are determined by the fat content of the milk. Inmost retail markets, dairy processors standardize milk to various levelsof fat, e.g., 1%, 2% and 3.25% in the United States. Also in cheeseproduction, quality and yield can be optimized by standardizing theratio of casein to fat in the milk used to make the cheese.Consequently, there is a need for an effective means of measuring fatand casein concentrations in milk. Standardization requires a robustmeasurement suitable for operation on the dairy production floor forprocess control applications.

[0005] Several automated techniques have been developed for measuringfat concentration in milk. Currently, measurement of the opticalturbidity of milk provides the most accurate and stable instrument-basedfat measuring technique. Simple broadband light attenuation measuringdetectors are used. The milk sample is homogenized and is diluted with ahigh pH diluent, such as sodium hydroxide, to dissolve calcium caseinatefrom the milk, so that it does not interfere with the fat measurement.There are no accepted simple instrumentation methods for measuringcasein concentration in milk.

[0006] Light scattering is a known technique for characterizingparticles in a liquid. In a light scattering system, a liquid containingparticles is passed through a sample cell having windows. A light beamis directed through the liquid, and light scattered by the particles inthe liquid is analyzed to determine the characteristics of theparticles. In one prior art system, the liquid sample is surrounded byan array of detectors which collect laser light scattered by the sampleat different angles. In another prior art system, laser light scatteredby the sample at a predetermined angle passes through an annularaperture and is focused on a photomultiplier. A light scattering systemfor molecular characterization is disclosed in U.S. Pat. No. 5,305,073issued Apr. 19, 1994 to Ford, Jr.

[0007] All of the known prior art light scattering systems have had oneor more disadvantages, including an inability to measure theconcentration of relatively small particles in the presence ofrelatively large particles. Accordingly, there is a need for improvedmeasurement systems and methods for determining component particleconcentrations in a liquid, such as fat and casein concentrations in adairy product.

SUMMARY OF THE INVENTION

[0008] According to a first aspect of the invention, apparatus isprovided for measuring component concentration in a dairy product. Theapparatus comprises a light source for generating a polarized light beamhaving a direction of polarization, a scattering cell, positioned in thelight beam, for receiving a sample of the dairy product, a first lightdetector positioned at a first angle in a range of about 5° to about 45°with respect to the light beam and a second light detector positioned ata second angle in a range of about 130° to about 160° with respect tothe light beam. The scattering cell has a first window and a secondwindow, wherein the second window is farther from the light source thanthe first window, wherein a normal to an exterior surface of the secondwindow is at or near Brewster's angle with respect to the light beam andwherein the direction of polarization of the light beam is parallel to aplane defined by the light beam and the normal to the exterior surfaceof the second window. The first light detector detects scattered lightfrom a first component of the dairy product having relatively largeparticle sizes and generates a first detector signal that isrepresentative of concentration of the first component in the sample ofthe dairy product. The second light detector detects scattered lightfrom a second component of the dairy product having a relatively smallparticle sizes and generates a second detector signal that isrepresentative of concentration of the second component in the sample ofthe dairy product. The first component may comprise fat, and the secondcomponent may comprise casein.

[0009] In a preferred embodiment, the first angle is about 40° and thesecond angle is about 140°. Because of refraction effects in the firstand second windows, the actual scattering angles differ from thelaboratory angles which define the positions of the light detectors, asdescribed below. The light source may comprise a laser and a polarizingdevice.

[0010] According to another aspect of the invention, a method isprovided for measuring component concentration in a dairy product. Themethod comprises the steps of generating a polarized light beam having adirection of polarization, placing a sample of the dairy product in ascattering cell that is positioned in the light beam, detectingscattered light from a first component of the dairy product at a firstangle in a range of about 5° to about 45° with respect to the light beamand generating a first detector signal, and detecting scattered lightfrom a second component of the dairy product at a second angle in arange of about 130° to about 160° with respect to the light beam andgenerating a second detector signal. The scattering cell has a firstwindow and a second window, wherein the light beam is incident on thesecond window after passing through the sample of the dairy product,wherein a normal to an exterior surface of the second window is at ornear Brewster's angle with respect to the light beam and wherein thedirection of polarization of the light beam is parallel to a planedefined by the light beam and the normal to the exterior surface of thesecond window. The first detector signal is representative ofconcentration of the first component in the sample of the dairy product,and the second detector signal is representative of concentration of thesecond component in the sample of the dairy product.

[0011] According to a further aspect of the invention, apparatus isprovided for measuring component concentration in a liquid containingrelatively large particles and relatively small particles. The apparatuscomprises a light source for generating a polarized light beam having adirection of polarization, a scattering cell, positioned in the lightbeam, for receiving a sample of the liquid, a first light detectorpositioned at a first angle in a range of about 5° to about 45° withrespect to the light beam, and a second light detector positioned at asecond angle in a range of about 130° to about 160° with respect to thelight beam. The scattering cell has a first window and a second window,wherein the second window is farther from the light source than thefirst window, wherein a normal to an exterior surface of the secondwindow is at or near Brewster's angle with respect to the light beam andwherein the direction of polarization of the light beam is parallel to aplane defined by the light beam and the normal to the exterior surfaceof the second window. The first light detector detects scattered lightfrom a first component of the liquid having relatively large particlesizes and generates a first detector signal that is representative ofconcentration of the first component in the sample of the liquid. Thesecond light detector detects scattered light from a second component ofthe liquid having relatively small particle sizes and generates a seconddetector signal that is representative of concentration of the secondcomponent in the sample of the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a better understanding of the present invention, reference ismade to the accompanying drawings, which are incorporated herein byreference and in which:

[0013]FIG. 1 is a graph of scattered light intensity as a function ofscattering angle for relatively large particles and relatively smallparticles;

[0014]FIG. 2 is a simplified schematic representation of a measurementsystem in accordance with an embodiment of the invention;

[0015]FIG. 3 is a schematic representation of a light scattering system,illustrating a difficulty in light scattering measurement of fat andcasein in milk;

[0016]FIG. 4 is a top schematic view of an embodiment of a measurementsystem in accordance with the invention;

[0017]FIG. 5A is a partial side view of the measurement system of FIG.4;

[0018]FIG. 5B is a schematic side view of a first embodiment of thesecond window of the scattering cell, illustrating the orientation ofthe second window with respect to the light beam;

[0019]FIG. 5C is a schematic side view of a second embodiment of thesecond window of the scattering cell, illustrating the orientation ofthe second window with respect to the light beam.

[0020]FIG. 6 is a cross-sectional side view of an implementation of ascattering cell utilized in the measurement system of FIGS. 4 and 5;

[0021]FIG. 7 is a block diagram that illustrates an application of thesystem for measuring component concentration in dairy products inaccordance with the invention;

[0022]FIG. 8 is a top schematic view of a system for measuring componentconcentration in a liquid containing relatively large particles andrelatively small particles;

[0023]FIG. 9 is a graph of sample composition wherein caseinconcentration in percent is plotted on the vertical axis and fatconcentration in percent is plotted on the horizontal axis;

[0024]FIG. 10 is a graph that compares fat concentration in percentdetermined from light scattering measurements, plotted on the verticalaxis, to fat concentration in percent determined from dilution, plottedon the horizontal axis; and

[0025]FIG. 11 is a graph that compares casein concentration in percentdetermined from light scattering measurement, plotted on the verticalaxis, to casein concentration in percent determined from dilution,plotted on the horizontal axis.

DETAILED DESCRIPTION

[0026] A measurement system in accordance with the present inventionexploits the way in which the physical properties of differentcomponents in a sample affect light scattered by the sample. Forexample, fat and casein in milk have different size distributions. Fatglobule size distribution in raw bovine milk ranges from about 0.2 to 30micrometers in diameter. After homogenization, the fat globule size isreduced so that most fat globules are under 3 micrometers in diameter,yielding a size distribution from about 0.2 to 3 micrometers. Caseinoccurs naturally in micellular form in the size range from 10 to 200nanometers (0.01 to 0.2 micrometers) in diameter.

[0027] The intensity of scattered light is a function of particle size,light wavelength and polarization direction, scattering angle and sampleconcentration. The scattered intensity at a small angle is dependentalmost entirely on the concentration of the large fat globules in themilk sample, and the scattered intensity at a large angle is dependentprimarily on the concentration of the smaller casein micells.

[0028]FIG. 1 shows the relative intensity of scattered light as afunction of actual scattering angle for relatively large particles (3micrometers) and relatively small particles (200 nanometers). Curve 10represents the scattered light intensity from 3 micrometer particles,and curve 12 represents the scattered light intensity from 200 nanometerparticles at a light wavelength of 800 nanometers. It may be observedthat the relatively large particles exhibit a large variation, more than100,000 to 1, in scattered light intensity as a function of scatteringangle. By contrast, the relatively small particles exhibit a smallvariation, less than 10 to 1, in scattered light intensity as a functionof scattering angle. For example, at an actual scattering angle of 26°,the intensity of light scattered from large fat particles substantiallyexceeds the intensity of light scattered from small casein particles.The reverse is true at an actual scattering angle of 153°. Thus, byseparately measuring the scattered light at 40° and 140° as measured inthe laboratory (actual scattering angles of 26.80° and 153.20°,respectively), the fat and casein concentrations in the milk can bedetermined without the use of a chemical diluent. The actual scatteringangle differs from the scattering angle measured in the laboratory(laboratory scattering angle) because of refraction effects in thescattering cell windows.

[0029] A simplified schematic diagram of an embodiment of a measurementsystem in accordance with the invention is shown in FIG. 2. Themeasurement system is configured for measuring fat and caseinconcentrations in a dairy product. A scattering cell 20, represented inFIG. 2 by a first window 22 and a second window 24, contains a sample ofa dairy product, such as diluted milk, for measurement. The dairyproduct contains fat particles 26 and casein particles 28. The sample islocated between windows 22 and 24 and flows continuously throughscattering cell 20. Scattering cell 20 is discussed in more detailbelow. A laser 30 directs a laser beam 32 along an optical axis 34through scattering cell 20. Laser beam 32 has a direction ofpolarization perpendicular to the plane of FIG. 2. As described indetail below, windows 22 and 24 are oriented at Brewster's angle withrespect to the polarized laser beam 32. The laser beam 32 passes throughfirst window 22, through the sample, and through second window 24. Asused herein, the “first” window refers to the scattering cell windowthat is closest to laser 30 or other light source, and the “second”window refers to the scattering cell window that is farthest from laser30 or other light source. Thus, laser beam 32 is incident on the secondwindow after passing through the liquid sample.

[0030] The laser beam 32 passing through scattering cell 20 is scatteredby components of the dairy product sample. The angular distribution ofthe scattered light depends upon the properties of the particles in thesample, including their concentrations. In the embodiment of FIG. 2, afirst photodetector 40 is located at an angle of 40° with respect tooptical axis 32, and a second photodetector 42 is located at an angle of140° with respect to optical axis 32. First photodetector 40 detectsscattered light at an angle of 40° and provides a detector signal thatis representative of fat concentration in the liquid sample. Secondphotodetector 42 detects scattered light at an angle of 140° andprovides a detector signal that is representative of caseinconcentration in the liquid sample. As shown in FIG. 1, the scatteredlight intensity from fat is significantly greater than the scatteredlight intensity from casein at small angles, and the scattered lightintensity from casein is significantly larger than the scattered lightintensity from fat at large scattering angles. Accordingly, themeasurement system of FIG. 2 provides measurements of the concentrationsof fat and casein in the dairy product.

[0031] The separation between fat and casein measurements is incomplete.Thus, the detector signal from photodetector 40 at an angle of 40°contains a predominant component from fat and a lesser component fromcasein. Similarly, the detector signal from photodector 42 at an angleof 140° contains a predominant component from casein and a lessercomponent from fat. This is apparent from FIG. 1. The unprocesseddetector signals therefore provide approximations to the fat and caseinconcentrations. As described below, more accurate values of fat andcasein concentration can be obtained by processing the detector signals.

[0032] Generally, it is not possible to measure scattered light fromsmall particles, such as casein, at large angles in the presence oflarge particles, such as fat, using prior art scattering cells. Thereason is described with reference to FIG. 3, which is a schematicrepresentation of a light scattering measurement system utilizing aconventional scattering cell. Light from a light source 70 is directedalong an optical axis 72 through a scattering cell 74 containing a dairyproduct, including relatively large fat particles 76 and relativelysmall casein particles 78. A first window 80 and a second window 82 ofscattering cell 74 have surfaces oriented perpendicular to optical axis72. In the system of FIG. 3, the light beam from light source 70 ispartially reflected by glass-liquid interface 84 and glass-air interface86 of second window 82. The reflected light is scattered in a forwarddirection by fat particles 76 in the same direction as the lightscattered by casein particles 78 in a backward direction from the mainlight beam. The reflected light scattered by fat particles 76 is moreintense than the light scattered by casein particles 78 from the mainlight beam. For this reason, casein concentration cannot be measured inthe system of FIG. 3.

[0033] An embodiment of a light scattering measurement system inaccordance with the invention is shown in FIGS. 4, 5A and 6. FIG. 4 is aschematic top view of the measurement system, and FIG. 5A is a schematicpartial side view of the measurement system. FIG. 6 is a cross-sectionalside view of an implementation of the scattering cell. Like elements inFIGS. 4, 5A and 6 have the same reference numerals. It will beunderstood that the orientation of the system can be changed and thatthe top and side designations are arbitrary.

[0034] A laser 110 directs a laser beam 112 along an optical axis 114through a scattering cell 120. Scattering cell 120 includes a firstwindow 122, a second window 124 and a housing 130 (FIG. 6). Housing 130encloses a scattering cell volume 132 between first window 122 andsecond window 124. As indicated above, first window 122 is closest tolaser 110 and second window 124 is farthest from laser 110. Scatteringcell 120 contains a sample for measurement, including relatively largefat particles 140 and relatively small casein particles 142. As shown inFIG. 5A, laser beam 112 is directed through a polarizing device 150 anda beam splitter 152 before reaching scattering cell 120. As shown inFIG. 5A, polarizing device 150 causes laser beam 112 to have a verticaldirection of polarization 154, so that a polarized light beam isincident on scattering cell 120. A first photodetector 160 is orientedat an angle of 40° in a horizontal plane with respect to laser beam 112.A second photodetector 162 is oriented at an angle of 140° in thehorizontal plane with respect to laser beam 112. A third photodetector164 is oriented at 90° with respect to laser beam 112 for monitoring thelight intensity emanating from beam splitter 152. Photodetector 164functions as a reference detector to normalize fluctuations in lightintensity generated by laser 110.

[0035] Preferably, laser beam 112 is in a wavelength range from about780 nanometers to about 1100 nanometers for measuring fat and caseinconcentrations in dairy products. In one example of the measurementsystem, laser 110 is a 780 nanometer, 2.5 milliwatt, polarized diodelaser available from Melles-Griot. Although the laser provides apolarized laser beam, approximately 1% of the light has the wrongpolarization and a separate polarizing device may be required.

[0036] Photodetectors 160, 162 and 164 may be type S2386-8K availablefrom Hamamatsu. Photodetectors 160 and 162 are not limited to angles of40° and 140°, respectively, with respect to laser beam 112. Moregenerally, photodetector 160 may be located at a laboratory angle in arange of about 5° to 45° for measuring scattered light from fatparticles, and photodetector 162 maybe located at a laboratory angle ina range of about 130° to 160° for measuring scattered light from caseinparticles. As described above, photodetectors 160 and 162 do not providecomplete separation between fat scattering measurements and caseinscattering measurements. Lenses 166 and 168 form images of thescattering region on the active areas of photodetectors 160 and 162,respectively. The active areas of the preferred photodiodes are 5.8×5.8millimeters.

[0037] As shown in FIG. 6, scattering cell 120 includes first window122, second window 124 and housing 130, which enclose cell volume 132.The cell volume 132 thickness between first window 122 and second window124 is defined by spacers 170. The liquid being measured flows throughcell volume 132 from an inlet port 172 to an outlet port 174. Accordingto a further feature, the scattering cell 120 may include a window,shown schematically at 176, positioned at 90° with respect to light beam112, to permit measurement of 90° light scattering.

[0038] In one example of scattering cell 120, first window 122 andsecond window 124 each have a thickness of 0.15 inch and are fabricatedof fused silica. Cell volume 132 is a circular disk having a diameter of1.10 inch. The cell volume thickness between windows 122 and 124,defined by spacers 170, is 0.015 inch. Inlet port 172 and outlet port174 have diameters of 0.10 inch. It will be understood that thesescattering cell parameters and the system components identified aboveare given by way of example only and are not limiting as to the scope ofthe invention.

[0039] As indicated above, light scattered from relatively largeparticles at small angles is much more intense than light scattered atlarge angles. For example, a particle 3 micrometers in diameter scatterslight about 1,000 times more intensely at an angle of 20° than at anangle of 160°. For this reason, it is not possible to make accuratemeasurements of small particles at large scattering angles usingconventional light scattering cells when large particles are present.The reason is that, even if the surfaces of the cell windows haveanti-reflection coatings, a small amount of the main light beam isreflected at the glass-air and glass-liquid interfaces. The lightdetected at a large scattering angle is made up of two components: (1)light scattered at the large angle from the incident light beam, and (2)light scattered at a small angle from the reflected beam parallel to thefirst component, as illustrated in FIG. 3. Even if the total reflectedlight intensity is only 0.5% (the reflection from a glass-waterinterface is about 0.3%), the undesired scattered light in the aboveexample is five times as intense as the desired scattered light in ameasurement at a scattering angle of 160°.

[0040] The reflected light may be eliminated or nearly eliminated byusing a polarized light source and tilting at least the secondscattering cell window, the window that is farthest from the lightsource, at or near Brewster's angle with respect to the polarized lightbeam. As known in the art, Brewster's angle is the angle between a lightbeam incident on an interface between two materials and a normal to theinterface at which no reflection occurs. Brewster's angle, θ_(B), isdefined as tan θ_(B)=n₂/n₁, where n₁ and n₂ are the indices ofrefraction of the materials on opposite sides of the interface, with n₁corresponding to the medium through which the light beam is incident onthe interface.

[0041] As illustrated in FIG. 5A, first window 122 has an exteriorsurface 180 and an interior surface 182; and second window 124 has anexterior surface 184 and an interior surface 186. In general,reflections from the surfaces 180 and 182 of first window 122 do notcause a problem, because the reflections do not pass through the liquidsample in scattering cell 120 and thus are not scattered towardphotodetector 162. With respect to second window 124, light is reflectedfrom interior surface 186 and exterior surface 184, when these surfacesare not oriented at Brewster's angle. Typically, the reflections fromexterior surface 184 are more intense than the reflections from interiorsurface 186, because the indices of refraction at the glass-airinterface (exterior surface 184) differ by more than the indices ofrefraction at the glass-liquid interface (interior surface 186).

[0042] In a first embodiment, the interior surface 186 and the exteriorsurface 184 of second window 124 are parallel, and window 124 is mountedat or near Brewster's angle with respect to light beam 112 for theglass-air interface of surface 184. The orientation of window 124 withrespect to light beam 112 is illustrated in the schematic diagram ofFIG. 5B. Light beam 112 is incident on surface 184 at Brewster's angleθ_(B) with respect to a normal 190 to exterior surface 184. Brewster'sangle θ_(B) is determined as described above with respect to exteriorsurface 184, a glass-air interface. In this embodiment, second window124 is preferably tilted at an angle θ_(B) of 55.463°, Brewster's anglefor a fused silica-air interface. Second window 124 is further orientedsuch that the direction of polarization 154 of light beam 112 isparallel to a plane defined by light beam 112 and normal 190 to exteriorsurface 184 of second window 124. This configuration ensures little orno reflection of light beam 112 from exterior surface 184. Forconvenience, first window 122 may be mounted parallel to second window124. However, first window 122 may have other orientations within thescope of the invention.

[0043] Because exterior surface 184 is a glass-air interface andinterior surface 186 is a glass-liquid interface, Brewster's angle isdifferent for the two surfaces. Thus, when exterior surface 184 isoriented at or near Brewster's angle with respect to light beam 112,interior surface 186 is not oriented at Brewster's angle with respect tolight beam 112, and a small fraction of light beam 112 may be reflectedfrom interior surface 186. However, it has been found that thereflections from the glass-liquid interface of surface 186 are small anddo not interfere to a significant degree with casein concentrationmeasurements at a scattering angle of 140°.

[0044] In a second embodiment, both exterior surface 184 and interiorsurface 186 of second window 124 are oriented at or near Brewster'sangle for the respective interfaces. As noted above, Brewster's angle isdifferent for the interfaces at surfaces 184 and 186. In order to insurethat each surface is oriented at the respective Brewster's angle, secondwindow 124 is fabricated with a wedge shape. A wedge-shaped secondwindow 124′ is shown in FIG. 5C. Exterior surface 184′ and interiorsurface 186′ of second window 124′ diverge at a wedge angle θ_(W) thatis equal to the difference in Brewster's angle for the interfaces atsurfaces 184′ and 186′. In the example of a fused silica window withwater as the fluid, the wedge angle θ_(W) is 7.997°. The wedge angleθ_(W) is exaggerated in FIG. 5C for illustrative purposes. Thus, secondwindow 124′ is oriented such that a normal 192 to exterior surface 184′is at or near Brewster's angle θ_(B1) with respect to light beam 112 forexterior surface 184′, and a normal 194 to interior surface 186′ is ator near Brewster's angle θ_(B2) with respect to light beam 112 forinterior surface 186′. As in the first embodiment, the direction ofpolarization 154 of light beam 112 is parallel to a plane defined bylight beam 112 and the normal 192 to exterior surface 184′ of secondwindow 124′. Using the wedge-shaped window 124′ with both surfacesoriented at Brewster's angle, light beam 112 is not reflected frominterior surface 186′ or exterior surface 184′. In the secondembodiment, first window 122 may be wedge-shaped for convenience ofmanufacturing or may have parallel surfaces. In general, it is notnecessary to orient first window 122 at Brewster's angle. However, theuse of two wedge-shaped windows may facilitate manufacturing of thescattering cell.

[0045] By tilting the windows of the sample cell 120 at or nearBrewster's angle, as described above, little or no light is reflectedback into the cell. The scattered light from the casein in the sample ismeasured at 140° without interference and is proportional to the caseinconcentration. A signal proportional to fat concentration is measured at40°. Thus, referring again to FIG. 4, photodetector 160 provides adetector signal that is representative of fat concentration in thesample, and photodetector 162 provides a detector signal that isrepresentative of casein concentration. As described above,photodetectors 160 and 162 do not provide complete separation betweenfat scattering measurements and casein scattering measurements.

[0046] Data analysis to determine fat and casein concentrations dependson the sample concentration. Sufficiently dilute samples generate singlescattering (light rays scatter only once before exiting the cell) andproduce a light intensity signal which increases linearly with componentconcentration. Fat and casein sample concentration are each related tomeasured light intensity at 40° and 140°, respectively, by Y=mX+b, whereX is the measured light intensity, and m and b are calibrationcoefficients for the specific system.

[0047] More concentrated samples generate multiple scattering, whereinlight rays scatter from more than one particle before exiting the cell.Component concentration is given by a polynomial equation in which thehigher order terms account for multiply scattered light. Fat and caseinconcentrations may each be calculated from measured light intensity at40° and 140°, respectively, by Y=aX+bX²+c, where X is the measured lightintensity, and a, b, and c are calibration coefficients for the specificsystem.

[0048] Since fat contributes to some of the scattered light measured at140° due to both multiple and single scattering, and casein contributesto some of the scattered light measured at 40° for the same reasons,more accurate results may be calculated using polynomial expressions ofmultiple variables. For example, fat concentration may be calculatedfrom F=aX+bX²+cZ+dZ²+e, where X is the light intensity measured at 40°,Z is the light intensity measured at 140°, and a, b, c, d, and e, arecalibration coefficients for the specific system. Casein concentrationmay be calculated from C=fX+gX²+hZ+iZ²+j, where X is the light intensitymeasured at 40°, Z is the light intensity measured at 140°, and f, g, h,i and j are calibration coefficients for the specific system. Othermathematical forms which account for the non-linearity of specificimplementations will yield optimum results for those implementations.The calibration coefficients may be determined experimentally for aparticular system.

[0049] The calibration coefficients may be determined by the followingsteps.

[0050] 1. Obtain two samples with known casein and fat concentrations,one with high fat concentration and low casein concentration, and theother with low fat concentration and high casein concentration.

[0051] 2. Make a number of mixtures of the two samples to obtain secondsamples with a range of concentrations of fat and casein.

[0052] 3. Make light scattering measurements on the second samples.

[0053] 4. Use standard curve fitting techniques to determine thecalibration coefficients from the results of the scatteringmeasurements.

[0054] This process is illustrated in FIGS. 9-11. FIG. 9 shows thecasein and fat concentrations for twenty different samples. The caseinconcentration in percent is plotted on the vertical axis, and the fatconcentration in percent is plotted on the horizontal axis. FIG. 10shows the fat concentrations determined from light scatteringmeasurements compared to fat concentrations determined from dilutions.In FIG. 10, fat concentration in percent determined from lightscattering measurements is plotted on the vertical axis, and fatconcentration in percent determined from dilution is plotted on thehorizontal axis. FIG. 11 shows the casein concentrations determined fromlight scattering measurements compared to casein concentrationsdetermined from dilutions. In FIG. 11, casein concentration in percentdetermined from light scattering measurements is plotted on the verticalaxis, and casein concentration in percent determined from dilution isplotted on the horizontal axis.

[0055] A simplified block diagram of an example of an application of themeasurement system of the present invention in the dairy processingindustry is shown in FIG. 7. FIG. 7 may represent a part of a cheesemaking facility. A system of the type shown in FIGS. 4-6 and describedabove is utilized. Like elements in FIGS. 4-7 have the same referencenumerals. A conduit 200 carries a dairy product, such as milk, for usein the process. A sample of the dairy product is diverted through asmaller conduit 202 to a first inlet of a mixing valve 204. A watersupply 210 provides water to a second inlet of mixing valve 204. Mixingvalve 204 provides the ability to dilute the dairy product with water tofacilitate light scattering measurements. In a preferred embodiment,milk is diluted with eight parts water to one part milk to provide asuitable sample for light scattering measurements. However, it will beunderstood that different dilution ratios may be utilized within thescope of the invention.

[0056] The diluted milk is provided as a liquid sample through inletport 172 of scattering cell 120. The liquid sample passes throughscattering cell 120 and outlet port 174 and is discarded aftermeasurement. Light scattering measurements of fat concentration are madeby photodetector 160 at a scattering angle of 40°, and light scatteringmeasurements of casein concentration are made by photodetector 162 at ascattering angle of 140°, as described above. The detector signals aresupplied to a process controller 220, which may comprise a personalcomputer (PC) or other computer or dedicated process controller. Theprocess controller 220 analyzes the measured fat and caseinconcentrations and generates a control signal based on the fat andcasein concentrations. In one example, the control signal varies the fatconcentration in the dairy product passing through conduit 200 toachieve a desired casein to fat ratio. It will be understood thatdifferent control functions may be based on the measured fat and caseinconcentrations.

[0057] A measurement system for determining fat and caseinconcentrations in a dairy product is described above in connection withFIGS. 2, 4, 5A, 6 and 7. However, a system of the type described abovemay be utilized more generally for measuring particle concentrations ina liquid containing relatively large particles and relatively smallparticles.

[0058] A schematic top view of a measurement system in accordance withan embodiment of the invention is shown in FIG. 8. A light source 310directs a polarized light beam 312 through a scattering cell 320. Thelight source 310 may be a laser or any other light source that producesa collimated beam. If necessary, the light source includes a polarizingdevice, such as a polarizing filter, to generate polarized light beam312. Scattering cell 320 is defined by a first window 322 and a secondwindow 324. A housing (not shown) encloses a cell volume between windows322 and 324. At least the exterior surface of second window 324 isoriented at or near Brewster's angle with respect to light beam 312, asdescribed above. Scattering cell 320 contains a liquid sample, includingrelatively large particles 330 and relatively small particles 332.

[0059] A first photodetector 350 at an angle of less than 90° withrespect to light beam 312 detects scattered light from large particles330, and a second photodetector 352 at an angle greater than 90° detectsscattered light from small particles 332. As described above,photodetectors 350 and 352 do not provide complete separation betweenfat scattering measurements and casein scattering measurements.Photodetectors 350 and 352 are located in a horizontal plane that passesthrough light beam 312, where light beam 312 has a vertical direction ofpolarization. As indicated above, the orientation of the entiremeasurement system may be varied within the scope of the invention. Therange of scattering angles available for measurement is limited becausethe scattered light must pass either through the first window 322 forlarge scattering angles or the second window 324 for small scatteringangles. The accessible scattering angle ranges are typically 5° to 45°in the forward direction and 130° to 160° in the backward direction, asmeasured in the laboratory. Thus, detector 350 is positioned within arange of 5° to 45° with respect to light beam 312, and photodetector 352is positioned within a range of 130° to 160° with respect to light beam312. Because of refraction effects in windows 322 and 324, the actualscattering angles are 3.44° to 29.92°, corresponding to laboratoryangles of 5° to 45°, and 147.03° to 166.33°, corresponding to laboratoryangles of 130° to 160°.

[0060] In the measurement systems described above, measurements of lightscattering from relatively large particles, such as fat particles, andmeasurements of light scattering from relatively small particles, suchas casein particles, may be made simultaneously or at different times.Furthermore, the measurement systems may be configured for measuring oneof the particle types. In the measurement system of FIG. 4,photodetector 162 may be utilized for measurements of caseinconcentration, and photodetector 160 may be omitted if measurement offat concentration is not required. Alternatively, photodetector 160 maybe utilized for measurements of fat concentration, and photodetector 162may be omitted if measurement of casein concentration is not required.Similarly, in the measurement system of FIG. 8, photodetector 350 orphotodetector 352 may be omitted when measurement of the respectiveparticle concentration is not required. In each case, the disclosedmeasurement system provides the capability to measure particleconcentration in a liquid containing both relatively large particles andrelatively small particles.

[0061] The system for measuring component concentration in dairyproducts shown in FIG. 7 utilizes a water supply 210 and a mixing valve204 for diluting the dairy product with water to facilitate lightscattering measurements. As stated above, in one embodiment ofscattering cell 120, the spacing between windows 122 and 124 is 0.015inch (FIG. 6).

[0062] In another embodiment, the spacing between windows 122 and 124 ofscattering cell 120 is selected to permit detection of light scatteredfrom particles of the sample of the dairy product without the need fordilution of the sample. The spacing between the first and second windowsis measured between the inside surfaces of the windows and may beestablished by spacers 170. In some embodiments, the spacing between thefirst and second windows is in a range of about 30 to 80 micrometers topermit detection of light scattered from particles of the sample of thedairy product without the need for dilution of the sample. In onespecific embodiment, the spacing between the first and second windows isabout 40 micrometers. By selecting the spacing between windows of thescattering cell such that dilution of the sample of the dairy product isnot required, mixing pumps, problems of air in the mixing water,problems of obtaining air-free water, and the inaccuracies associatedwith these problems, are eliminated. Thus for example, in the embodimentof FIG. 7, water supply 210 and mixing valve 204 may be eliminated.

[0063] While there have been shown and described what are at presentconsidered the preferred embodiments of the present invention, it willbe obvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

What is claimed:
 1. Apparatus for measuring component concentration in adairy product, comprising: a light source for generating a polarizedlight beam having a direction of polarization; a scattering cell,positioned in the light beam, for receiving a sample of the dairyproduct, said scattering cell having a first window and a second window,wherein the second window is farther from said light source than thefirst window, wherein a normal to an exterior surface of the secondwindow is at or near Brewster's angle with respect to the light beam andwherein the direction of polarization of the light beam is parallel to aplane defined by the light beam and the normal to the exterior surfaceof the second window; a first light detector, positioned at a firstangle in a range of about 5° to about 45° with respect to the lightbeam, for detecting scattered light from a first component of the dairyproduct having relatively large particle sizes and generating a firstdetector signal that is representative of concentration of the firstcomponent in the sample of the dairy product; and a second lightdetector, positioned at a second angle in a range of about 130° to about160° with respect to the light beam, for detecting scattered light froma second component of the dairy product having relatively small particlesizes and generating a second detector signal that is representative ofconcentration of the second component in the sample of the dairyproduct.
 2. Apparatus as defined in claim 1, wherein said first angle isabout 40° and wherein said second angle is about 140°.
 3. Apparatus asdefined in claim 1, wherein said light source comprises a laser and apolarizing device.
 4. Apparatus as defined in claim 3, wherein saidlight beam has a wavelength in a range of about 780 to 1100 nanometers.5. Apparatus as defined in claim 1, wherein the first window and thesecond window of said scattering cell have parallel surfaces. 6.Apparatus as defined in claim 1, wherein said scattering cell includesan inlet port and an outlet port to permit the dairy product to flowthrough the scattering cell.
 7. Apparatus as defined in claim 1, whereinan interior surface of the second window is parallel to the exteriorsurface of the second window.
 8. Apparatus as defined in claim 1,wherein a normal to an interior surface of the second window contactingthe sample of the dairy product is at or near Brewster's angle withrespect to the light beam, wherein the second window is wedge-shaped. 9.Apparatus as defined in claim 1, wherein said light source comprises alaser diode that generates said polarized light beam.
 10. Apparatus asdefined in claim 1, further comprising means for diluting the dairyproduct with water to provide the sample of the dairy product formeasurement.
 11. Apparatus as defined in claim 1, wherein a spacingbetween the first and second windows in selected to permit the detectionof light scattered from particles in the sample of the dairy productwithout dilution thereof.
 12. Apparatus as defined in claim 1, wherein aspacing between the first and second windows is in a range of about 30to 80 micrometers.
 13. Apparatus as defined in claim 12, wherein thespacing between the first and second windows is about 40 micrometers.14. Apparatus for measuring component concentration in a liquidcontaining relatively large particles and relatively small particles,comprising: a light source for generating a polarized light beam havinga direction of polarization; a scattering cell, positioned in the lightbeam, for receiving a sample of the liquid, said scattering cell havinga first window and a second window, wherein the second window is fartherfrom said light source than the first window, wherein a normal to anexterior surface of the second window is at or near Brewster's anglewith respect to the light beam and wherein the direction of polarizationof the light beam is parallel to a plane defined by the light beam andthe normal to the exterior surface of the second window; a first lightdetector, positioned at a first angle in a range of about 5° to about45° with respect to the light beam, for detecting scattered light from afirst component of the liquid having relatively large particle sizes andgenerating a first detector signal that is representative ofconcentration of the first component in the sample of the liquid; and asecond light detector, positioned at a second angle in a range of about130° to about 160° with respect to the light beam, for detectingscattered light from a second component of the liquid having relativelysmall particle sizes and generating a second detector signal that isrepresentative of concentration of the second component in the sample ofthe liquid.
 15. Apparatus as defined in claim 14, wherein said lightsource comprises a laser and a polarizing device.
 16. Apparatus asdefined in claim 14, wherein said light beam has a wavelength in a rangeof about 780 to 1100 nanometers.
 17. Apparatus as defined in claim 14,wherein the first window and the second window of said scattering cellhave parallel surfaces.
 18. Apparatus as defined in claim 14, wherein aninterior surface of the second window is parallel to the exteriorsurface of the second window.
 18. Apparatus as defined in claim 14,wherein a normal to an interior surface of the second window contactingthe sample of the liquid is at or near Brewster's angle with respect tothe light beam, wherein the second window is wedge-shaped.
 20. Apparatusas defined in claim 14, wherein said light source comprises a laserdiode that generates the polarized light beam.
 21. A method formeasuring component concentration in a dairy product, comprising thesteps of: generating a polarized light beam having a direction ofpolarization; placing a sample of the dairy product in a scattering cellthat is positioned in the light beam, said scattering cell having afirst window and a second window, wherein the light beam is incident onthe second window after passing through the sample of the dairy product,wherein a normal to an exterior surface of the second window is at ornear Brewster's angle with respect to the light beam and wherein thedirection of polarization of the light beam is parallel to a planedefined by the light beam and the normal to the exterior surface of thesecond window; detecting scattered light from a first component of thedairy product, the first component having relatively large particlesizes, at a first angle in a range of about 5° to about 45° with respectto the light beam and generating a first detector signal that isrepresentative of concentration of the first component in the sample ofthe dairy product; and detecting scattered light from a second componentof the dairy product, the second component having relatively smallparticle sizes, at a second angle in a range of about 130° to about 160°with respect to the light beam and generating a second detector signalthat is representative of concentration of the second component in thesample of the dairy product.
 22. A method as defined in claim 21,wherein the step of detecting scattered light at a first angle comprisesdetecting scattered light at about 40° and wherein the step of detectingscattered light at a second angle comprises detecting scattered light atabout 140°.
 23. A method as defined in claim 21, further comprising thestep of diluting the dairy product with water to provide the sample ofthe dairy product.
 24. A method as defined in claim 21, furthercomprising the step of selecting a spacing between the first and secondwindows to permit detection of light scattered from particles in thesample of the dairy product without dilution thereof.
 25. A method formeasuring component concentration in a liquid containing relativelylarge particles and relatively small particles, comprising the steps of:generating a polarized light beam having a direction of polarization;placing a sample of the liquid in a scattering cell that is positionedin the light beam, said scattering cell having a first window and asecond window, wherein the light beam is incident on the second windowafter passing through the sample of the liquid, wherein a normal to anexterior surface of the second window is at or near Brewster's anglewith respect to the light beam and wherein the direction of polarizationof the light beam is parallel to a plane defined by the light beam andthe normal to the exterior surface of the second window; detectingscattered light from a first component of the liquid sample, the firstcomponent having relatively large particle sizes, at a first angle in arange of about 5° to about 45° with respect to the light beam andgenerating a first detector signal that is representative ofconcentration of the first component in the sample of the liquid; anddetecting scattered light from a second component of the liquid sample,the second component having relatively small particle sizes, at a secondangle in a range of about 130° to about 160° with respect to the lightbeam and generating a second detector signal that is representative ofconcentration of the second component in the sample of the liquid. 26.Apparatus for measuring component concentration in a dairy product,comprising: a light source for generating a polarized light beam havinga direction of polarization; a scattering cell, positioned in the lightbeam for receiving a sample of the dairy product, said scattering cellhaving a first window and a second window, wherein the second window isfarther from said light source than the first window, wherein a normalto an exterior surface of the second window is at or near Brewster'sangle with respect to the light beam and wherein the direction ofpolarization of the light beam is parallel to a plane defined by thelight beam and the normal to the exterior surface of the second window;a first light detector, positioned at a first angle in a range of about5° to about 45° with respect to the light beam, for detecting scatteredlight from a first component of the dairy product comprising fat havingrelatively large particles and generating a first detector signal thatis representative of concentration of the first component in the sampleof the dairy product; a second light detector, positioned at a secondangle in a range of about 130° to about 160° with respect to the lightbeam, for detecting scattered light from a second component of the dairyproduct comprising casein having relatively small particles andgenerating a second detector signal that is representative ofconcentration of the second component of the sample of the dairyproduct; and a data analyzer, responsive to the first and seconddetector signals, for generating a signal representative of a casein tofat ratio in the sample of the dairy product, wherein a spacing betweenthe first and second windows is selected to permit detection of lightfrom particles in the sample of the dairy product without dilutionthereof.
 27. Apparatus as defined in claim 26, wherein the spacingbetween the first and second windows is in a range of about 30 to 80micrometers.
 28. Apparatus as defined in claim 26, wherein the spacingbetween the first and second windows is about 40 micrometers. 29.Apparatus as defined in claim 26, wherein said first angle is about 40°and wherein said second angle is about 140°.